Crucibles

ABSTRACT

The invention provides an improved design for a crucible for the melting of materials by means of radio frequency eddy currents induced in the materials. A segmented platform is located intermediate an open end of the crucible and a closed base at the opposite end, in a region where the crucible wall is segmented, thereby to separate a melt from the &#39;&#39;&#39;&#39;cold spot&#39;&#39;&#39;&#39; in the crucible caused by the electrical short circuit of the closed base end. The outer surface of the wall segments is curved and defines a single cylindrical surface to facilitate fitting within a primary induction coil.

United States Patent Hukin "[15; 3,702,368 [451 Nov. 7, 1972 1 CRUCIBLES[72] Inventor: David Ainsworth Hukin, The Cotgland 22 Filed: Dec.30,1970 21 Appl.No.: 102,608

tage, 20 Manor Road, Oakley, En-

[30] Foreign Application Priority Data Jan. 9, 1970 [52] U.S.Cl ..l3/26, 2l9/-10.49'

[51] Int. Cl. ..H05b 5/16 [58] Field of Search ..l3/26, 27; 219/1049[56] References Cited UNITED STATES PATENTS Great Britain .'...;1,139/70Seale ..13/27 3,461,2l5 8/1969 Reboux ..l3/27 3,520,980

7/ 1970 Sterling et al ..l3/27 Primary Examiner-Roy N. Envall, Jr.Attorney-Beveridge & De Grandi [57] I 7 ABSTRACT The invention providesan improved design for a crucible for the melting of materials by meansof radio frequency eddy currents induced in the materials. A segmentedplatform is located intermediate an open .end of the crucible and aclosed base at the opposite end, in a region where the crucible wall issegmented, thereby to separate a melt from the cold spot in the cruciblecaused by the electrical short circuit of the closed base end. Theoutersurface of the wall segmentsis curved and definesa single cylindricalsurface 'to fa'cilitate fitting within a primary induction coil.

' 7 Claims, 2 Drawing Figures 1 CRUCIBLES The present invention relatesto crucibles for the melting of materials by means ,of radio. frequencyeddy currents induced in the materials. j

' The melting of fusible materials in metal crucibles by means of radiofrequency eddy currents induced within the materials is a well knowntechnique which has wide applications in. the fields of metallurgy. andsemi conductortechnology. Conventionally, the eddy currents aregenerated by means of a primary, or work, coil surrounding the crucible,to which radio'frequency power is supplied. Generally metal crucibleshave been of segmented wall construction to reduce eddy current lossesin the crucible walls, Thus a common crucible'design sel having a thickwall, the outer surface of which is in the form of at least part of acylinder and which has an inner surface whose form is such as to provideat at Y least oneend of the vessel a region of reduced crossconsists ofa'generally c upshaped array of tubes of cir-' =cular cross-sectionpositioned sufficiently closely together for molten materialto beretained in the c'rucible by surface tension effects. The tubes areconnected together at their ends to enable cooling fluid to becirculated through them. However, such crucibles suffer from thedisadvantage that the manifolds connecting together the ends of thetubes short-circuit them, thus reducing the electric field in theseregions so causing the .ends of thecrucible to be considerably colder.than the central region. This can prevent the setting up of desiredtemperature conditions within the molten material, for example, ifsingle crystal material is to be made from the molten material.

Furthermore the'circular cross-section of the tubes does not permit veryclose electrical coupling of the work coil to the crucible and thecrucible to its load. Thus, high power radio frequency generators arerequired to overcome the losses in the crucible.

It is an object of the present invention to provide an improvedcruciblein which cold regions thereof do not affect the temperatureconditions within a melt and capable of close coupling to a primaryinduction coil.

According to the present invention a segmented hollow wall crucible-openat one end to receive material forprocessing and closed at the other bya base which alone joins the wall segments, is provided with a segmentedplatform intermediate said open end and said base for supportingmaterial remote from said base.

. Preferably the platform segments are integral inwardly'directedportions of the crucible wall segments.

In order to facilitate coupling with a primary induction coil the outersurfaces of the wall segments preferably all lie on a single cylindricalsurface.

Conveniently the base includes a set of ports each communicating withthe hollow interior of a wall segbase by which it can be screwed onto asupporting spigot having a complementary thread profile and, extendingfrom a platform which cooperates with the underside of the base to forma cooling water reservoir. It isenvisaged that a crucible according tothe invention will be fitted with a primary induction coilembracsectional area, the said wall being interrupted by a plurality ofslots extending parallel to the axis of the cylinder, the said slotsextending throughout and beyond the region or regions of reduced area,the arrangement also being such that cooling fluid can be'circulatedthrough the hollowmembers.

In a preferred embodiment of the invention the inner surface has a formsuch as to provide anenclosure in the shape of a cup with a taperinginterior having a central aperture. The crucible isintended for use witha radio frequency heating coil arranged to extend beyond the platformformed by the region of reduced'crossfrequency heating coil means thatefiectively, so far as the radio frequency field is concerned, theindividual members forming the walls of the crucible are notshort-circuited together, each forming a separate transformer for theradio frequency currents.

Suitable materials for the crucible are high purity copper, or silver.The surfaces of the crucible may be plated with some reflective metal toreduce heat losses ing at least that portion of the crucible betweensaid open end and the platform. 7 The present invention thus provides acrucible comprising a series of hollow members of high electrical andthermal conductivity so shaped as to provide a vesfrom the inside of thecrucible.

Theinvention will be further described by way of example with referenceto theaccompanying drawings in which: r r

I FIG. 1 is a longitudinal section of an embodiment of the invention,and l V p FIG. 2 is a perspective view, partly cut-away, of theembodiment of FIG. 1. I

, Referring to thedrawings, a crucible for use as a socalled coldcrucible for the radio frequency melting of materials, consists of anumber of elements 1 of trapezoidal cross-section separated by gaps 2which are formed by cutting radial slots in a solid bar ofhigh-conductivity copper 3 of some 2 A inch diameter. A bore 4 of 1 34inch diameter and having a hemispherical bottom is formed in the copperbar 3 at one end, resulting in a crucible having walls of some inchthickness, a cylindrical external surface S and a cup-shaped internalsurface 6. In the other end of the bar 3 is a counterbore 7 of somewhatless diameter than the cup-shaped bore 4. Beyond the counterbore 7 thesolid metal forms a a base 4a. Where the slots meet at the bottom of thecupshaped bore 4 a hole 8 is formed of some /4 inch diameter in a region9 of the vessel. The size of the hole 8 is not critical and is in factcontrolled by the width of the slots, the narrower the slots the smallerthe hole 8. The region of the side wall between the upper and lower Ibores 4 and 7, respectively, acts as a concentrator for the radiofrequency field ensuring that the bottom of the crucible is at least ashot as the major partof the side wall. The region 9 also shapes theradio frequency field so that the material is levitated in the crucible.Each of the elements 1 forming the wall of the crucible is boredlongitudinally, each bore la connects via a port 10 of a first manifold,and has a tube 11 inserted in it which connects via a port 12 in asecond manifold formed in the bar3 to enable cooling water to becirculated throughout the wall segments of the crucible. A screw thread13 enables the crucible to be screwed on to a spigot 15 of supportingstand 17, which can be utilized to form part of the cooling system. Tothis end the spigot includes bores 18, 19 for communicating between awater supply and the first manifold ports 10 and thestand has bores 20,21 for communicating between the water supply and the second manifoldports 12; The crucible is used in conjunction with a primarycoil 14which is arranged to extend from the top of l the crucible to a pointsomewhat below the beginning ofthe counterbore 7. Thus each segment 1 iselectrically isolated from its neighbors in the region of the radiofrequency field and functions independently as a transformer, and theelements 1 form a cup-shaped crucible which is closely coupled to acharge within it over virtually the whole of its surface, including thebottom, and is closely coupled to the primary coil. The large numbers ofelements 1, (16 in all), and the symmetrical shape of the crucible givea stable melt even though a low frequency ripple may be present on thehigh tension supply to the primary coil.

' The fact that the crucible is supported by the stand in a verticalposition means that there is no obstruction at the open end of thecrucible to interfere with the installation of ancillary equipment suchas radiation shields, receivers for the vacuum distillation of metalsfrom the crucible; or equipment for the pulling of single crystals froma melt contained in the crucible.

The merits of the invention are illustrated by the facts that using theabove described crucible a radio frequency power input of some 15 kWsufficed to completely melt in air a charge of 120 gm. of platinum and acharge of 60 gm. of lanthanum was completely melted in vacuo by 6 kW ofradio frequency power.

I claim:

l. A crucible having a generally cylindrical body including a pluralityof elongated slots extending longitudinally in the body andcircumferentially spaced about the longitudinal axis of the body todefine a plurality of elongated wall segments of the body, the bodybeing hollow to provide a melt chamber surrounded and defined by saidwall segments, said wall segments terminating in free extremities spacedfrom each other at one end of the body, said slots and wall segmentsdefined thereby terminating intermediate the ends of the body with theportion of said body between the wall segments and other end of the bodyconstituting a base integral with said wall segments, an induction coilmounted about said body in surrounding relation to said wall segments,the cross-section of said wall segments gradually and uniformlyincreasing in thickness in the transverse direction of the body at aregion opposite the free extremities of: said wall segments to define asegmented platform extending transversely of the body intermediate theends thereof to concentrate a radio frequency field on said: regionwhile also shaping the radio frequency field at said re ion, said inductron coil extending from ad acent sai one end of the body to saidregion. 0

2. A crucible as set forth in claim 1 in which the outer surfaces of thewall segments all lie on a single cylindrical surface to facilitatelocation of a primary induction coil therearound.

3. A crucible as set forth in claim 3 in which the radially innermostends of the platform segments all lieon a coaxial cylindrical surface.

4. A crucible as set forth in claim 1 in which the base is cut away todefine a first set of ports each communicating with the hollow interiorof a wall segment and a second set of ports each communicating with oneend of a tube fitted within each hollow wall segment and communicatingwith the remote end thereof.

5. A crucible as set forth in claim 1 in which the base is cut away toform a central threaded opening by which it can be screwed onto asupporting spigot having a complementary thread profile.

6. A crucible as set forth in claim 5 in combination with a supportingplatform having an upstanding externally threaded spigot for engagementin the threaded opening in the base.

7. A crucible and platform combination'as set forth in claim 6 whereinthe platform cooperates with the underside of the base to form a coolingwater reservoir.

1. A crucible having a generally cylindrical body including a pluralityof elongated slots extending longitudinally in the body andcircumferentially spaced about the longitudinal axis of the body todefine a plurality of elongated wall segments of the body, the bodybeing hollow to provide a melt chamber surrounded and defined by saidwall segments, said wall segments terminating in free extremities spacedfrom each other at one end of the body, said slots and wall segmentsdefined thereby terminating intermediate the ends of the body with theportion of said body between the wall segments and other end of the bodyconstituting a base integral with said wall segments, an induction coilmounted about said body in surrounding relation to said wall segments,the cross-section of said wall segments gradually and uniformlyincreasing in thickness in the transverse direction of the body at aregion opposite the free extremities of said wall segments to define asegmented platform extending transversely of the body intermediate theends thereof to concentrate a radio frequency field on said region whilealso shaping the radio frequency field at said region, said inductioncoil extending from adjacent said one end of the body to said region. 2.A crucible as set forth in claim 1 in which the outer surfaces of thewall segments all lie on a single cylindrical surface to facilitatelocation of a primary induction coil therearound.
 3. A crucible as setforth in claim 3 in which the radially innermost ends of the platformsegments all lie on a coaxial cylindrical surface.
 4. A crucible as setforth in claim 1 in which the base is cut away to define a first set ofports each communicating with the hollow interior of a wall segment anda second set of ports each communicating with one end of a tube fittedwithin each hollow wall segment and communicating with the remote endThereof.
 5. A crucible as set forth in claim 1 in which the base is cutaway to form a central threaded opening by which it can be screwed ontoa supporting spigot having a complementary thread profile.
 6. A crucibleas set forth in claim 5 in combination with a supporting platform havingan upstanding externally threaded spigot for engagement in the threadedopening in the base.
 7. A crucible and platform combination as set forthin claim 6 wherein the platform cooperates with the underside of thebase to form a cooling water reservoir.